Method and apparatus for sound and vibration detection

ABSTRACT

A triple-axis shockproof and waterproof vibration transducer is disclosed for detecting vibrations in general and sound from a musical instrument in particular, especially one having a resonant cavity. The transducer includes three piezoelectric vibration detectors, each oriented to detect vibrations along a different one of three orthogonal axes established by walls of a sealed enclosure filled with silicon rubber. All detectors are connected in parallel to the input of a noninverting operational amplifier which adds the separate voltages produced proportional to vibrations along the three orthogonal axes. A universal wax is employed to attach the transducer to the optimum point of the instrument as determined empirically.

United States Patent [72] Inventor Arnold Lazarus 3,453,920 7/1969Scherer 84/116 43 Dore St., San Francisco, Calif. 94103 3,538,23211/1970 Bachtig et a1. 3 l0/8.4 X [211 PP 12554 1 7 OTHER REFERENCES g:2; 5% Norman l-l. Crowhurst, Electronic Musical Instrument Handbook,pp. 37- 38, 112- 113, Howard W. Sams & Co., Inc., The Hobbs-Merrill Co.,Inc., Indianapolis, New York 54 METHOD AND APPARATUS FOR SOUND AND 214)VIBRATION DETECTION Primary Examiner-D. F. Duggan 13 Claims, 4 Drawlng gAssistant Examiner-Ulysses Weldon 52 us. Cl 84/1.l4, & Fteilich 84/1.16,310/8.4 E2 ABSTRACT: A triple-axis shockproof and waterproof vibra- I l3 t 21 T tion transducer is disclosed for detecting vibrations ingeneral and sound from a musical instrument in particular, especially73/67" 133 324/56 one having a resonant cavity. The transducer includesthree [56] References Cited piezoelectric vibration detectors, eachoriented to detect vibrations along a different one of three orthogonalaxes UNITE? STATES PATENTS established by walls of a sealed enclosurefilled with silicon "744916 1/ Nfcolsont 179/1 rubber. All detectors areconnected in parallel to the input of 2,280,226 4/1942 Firestone... anoninverting operational amplifier which adds the separate 2,728.86812/1955 Peterson 3 l0/8.4 voltages produced proportion to vibrationsakmg the three 23639 12/1960 ccfun'ney'pran at 310/84 X orthogonal axes.A universal wax is employed to attach the 338L149 4/1968 Wiggins 310/85X transducer to the optimum point of the instrument as deter- 3,437,8SI4/I969 Cady 310/85 X mined empirically I J Z 2 SILICON QUBBEZ FILLEDMETHOD AND APPARATUS FOR SOUND AND VIBRATION DETECTION BACKGROUND OF THEINVENTION This invention relates to a method and apparatus for detectingvibrations, and in particular for converting vibrations from musicalinstruments, especially ones having a resonant cavity, into highfidelity electrical signals for amplification and reproduction ofmusical sounds.

A variety of electromechanical transducers have been employed to produceelectrical signals from vibrations in general, and of musicalinstruments in particular. The most common has been a type employingpolarized ferroelectric (piezoelectric) ceramics for receivingvibrations, such as from a musical instrument.

While some attention has been given to the nature of mechanical couplingof a transducer to a musical instrument having a resonant cavity,sufficient attention has not been given to the point on the instrumentat which attachment should be made, nor to the nature of mechanicalmotion or vibration produced by the instrument of the point ofattachment. For example, it has been standard practice to attach atransducer to a stringed instrument at a point centered under thestrings, and near the string bridge. While such a point may be the mosteffective for producing an electrical signal which corresponds infrequency to the fundamental tone of a given note played, it may not bethe most effective point for producing an electrical signal which alsocorresponds to the overtones that produce the tonal qualities of themusical instrument. Moreover, once such a point of attachment has beenselected, it has been standard practice to provide some form ofpermanent attachment.

An object of this invention is to provide a method of converting amusical sound from an instrument that produces audible sound byvibrations in three orthogonal axes for amplification into an electricalsignal which corresponds in frequencies to the tone and the overtones tothe musical instrument one hears projected to him through the air.

Still another object is to provide a method of attaching a transducer ona variety of instruments without modification to the transducer or theinstrument.

Another object is to provide a transducer on a musical instrument thatresponds to vibrations in three orthogonal axes for producing anelectrical signal to a load that is the sum of separate electricalsignals, each proportional to vibrations in a different one of the threeaxes.

Another object is to provide a transducer on a musical instrument thatis effective in producing an electrical signal that includes frequenciesof a fundamental tone and overtones created by vibrations in threeorthogonal axes of a musical instrument.

OBJECTS AND SUMMARY OF THE INVENTION These and other objects of theinvention are achieved by three vibration detectors mounted on a commonbase and electrically connected to form a triple-axis transducer adaptedto be mechanically coupled to a vibrating object, such as a musicalinstrument, with adhesive wax. In a musical instrument having a resonantcavity, the point selected is one which reflects the same frequency andtonal qualities of the musical instrument as one hears projected to himfrom the instrument. That point is determined empirically by mountingthe transducer with universal wax that can be readily removed with asolvent. Each of the three vibration detectors is mechanically coupledto the common base and oriented to convert vibrations in a separate oneof three orthogonal axes into a proportional electrical signal.

Each of the vibration detectors comprises a piezoelectric (PZ) elementheld between a conductive film on the common base and an inertia masswith a thin layer of cement. The common base comprises at least threeorthogonal walls, one wall for each of the vibration detectors. A givendetector is so oriented as to produce an electrical signal in responseto the well-known piezoelectric effect by which an electric charge isproduced between the faces of the P2 element when pressure against thefaces changes due to vibrational motion of the wall to which attached ina direction normal to the wall. All three of the vibration detectors areelectrically connected in parallel with one end of each connected to asumming circuit comprising a high-input impedance operational amplifier.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims.

The invention will best be understood from the following descriptionwhen read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front view of a musicalinstrument representative of instruments having a resonant cavity forsound amplification and projection with a transducer attached inaccordance with the present invention.

FIG. 2 is an isometric view of a transducer according to the presentinvention partially broken away to show three detectors mounted fordetecting vibrations is three orthogonal axes.

FIG. 3 is an enlarged cross-sectional view of one detector mounted on awall of a common base according to the present invention.

FIG. 4 is an electrical diagram of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I of thedrawings, there is illustrated a guitar which is representative ofmusical instruments having resonant cavities for sound amplification andprojection.

Other instruments which also have a resonant cavity, and

therefore may also utilize the present invention for electronicallyamplifying music more exactly as it sounds than has herebefore beenpossible, are the violin, cello, bass viol, mandolin, harp, piano andAfrican thumb piano. Still others could acoustically mentioned; the listis given by way of example and not by way of limitation.

The guitar illustrated is a modern music guitar but is similar to anyother guitar in that it includes a body portion 10 which is hollow.Openings 11 and 12 on the face of the body portion 10 permit it tofunction as a resonant cavity for amplification and projection of sound.The term resonant cavity" is used herein to mean any hollow body havingan opening for the purposes of acoustically amplifying and projectingsound.

The guitar includes a fretted neck 13 and six strings held under tensionover the neck by tone-adjusting pegs, such as a string 14 held undertension by a peg 15. The other end of each of the strings is tied to atailpiece l6 anchored to an end 17 of the resonant cavity opposite theneck I3. A bridge 18 serves to hold the strings away from the body 10and the neck 13 so that they may be free to vibrate when strummed orplucked. The fundamental frequency with which a given string willvibrate is determined by its length from the bridge 18 to a point on theneck 13 against which the string is pressed by the musician.

The face of the resonant cavity will vibrate with the fundamentalfrequency of a note played. A note is thus amplified by the resonantcavity and, in the process, overtones are created by the interaction ofthe vibrating string with the resonating cavity. These overtones givethe note played a quality which is characteristic of the instrument.

In order to electronically amplify sound from a musical instrumenthaving a resonant cavity, such as the guitar illustrated in FIG. 1, ithas been the practice to mechanically couple to the resonant cavity atransducer comprising a vibration detector. The point at which theconnection has been customarily made is centered under the strings andnear the bridge. In addition, the vibration detector has, in the past,been oriented in a fixed point to detect vibration along a single axis,generally an axis normal to plane tangent to the face of the resonantcavity at the point where the detector is connected.

This prior art arrangement has provided efficient detection of vibrationat the frequency of the fundamental note played, but not of vibrationsat the frequencies of the overtones. It has been discovered that, toreproduce sound with high fidelity from the electrical signal (asthrough a loudspeaker), the signal from the transducer must includefrequency components of vibrations along three orthogonal axes and notmerely along one principal axis. An analogy can be drawn to a ship in astorm. A graph of vertical acceleration will not be nearly as indicativeof passenger discomfort as a graph that includes roll and pitch, namelya graph of the sum of acceleration along three orthogonal axes. In thecase of a single-axis transducer coupled to the resonant cavity of aninstrument, the vibrations along the other two axes may be small, butthey contribute significantly to the tonal qualities of the music onehears projected to him directly from the instrument. Therefore, inaccordance with the present invention, three orthogonally orienteddetectors are provided and their voltage outputs are added directly.

Referring next to FIG. 2, a triple-axis transducer 20 is shown fordetecting vibrations along three orthogonal axes in accordance with thepresent invention. The transducer includes three vibration detectors 2],22 and 23, each comprising a piezoelectric (PZ) accelerometer forproducing a voltage signal proportional to vibrations along one of threeorthogonal axes x, y and z, thus converting mechanical energy (sound)into electrical energy suitable for amplification. A housing for thethree vibration detectors consists of an epoxy shell 24 having at leastthree mutually perpendicular walls 25, 26 and 27, each with a copperfilm on the inside face electrically connected to all other copper filmsto provide a common support and a common conductor for the vibrationdetectors.

In practice, five copper-clad boards may be assembled to form an openbox. Once the vibration detectors have been cemented to three mutuallyperpendicular walls as shown, a sixth copper-clad board is electricallyconnected to the other copper-clad boards and placed as a lid over thebox. Altematively, a single copper-clad board may be cut and folded toform an open box with a lid hinged along one edge. In either case, thecopper film is produced on a board of dielectric material, such as epoxyimpregnated fiber glass cloth, in the same manner as for conventionalcircuit boards.

Before placing a lid on the assembly, an inner conductor 28 of a coaxialcable 29 is connected to each of the detectors, and the outer conductorof the cable 29 is electrically connected to the copper film on thewalls of the transducer. Then edges of the box, and the opening in thebox through which the coaxial cable 29 passes, are sealed with epoxy.Before sealing, the box is filled with silicon rubber, such as DowComing 3110 silastic. After sealing, the box assembly is provided withan epoxy shell using conventional epoxy potting techniques. The resultis a sensitive, shockproof and waterproof transducer capable of pickingup vibrations in three orthogonal axes without electrostatic noise orhum and without acoustic feedback since all of the vibration detectorsare shielded and are sensitive to only vibrations of the resonant cavityof the instrument. The silicon rubber fill contributes significantly tothe shockproof and waterproof qualities of the transducer withoutsignificantly degrading sensitivity.

Before describing the present invention in greater detail with referenceto an electrical diagram in FIG. 4, a given vibration detector will bedescribed with reference to FIG. 3. The manner of mechanically couplingthe transducer 20 to the musical instrument will also be describedfirst.

Referring now to FIG. 3, which illustrates a cross-sectional view of thevibration detector 22 of FIG. 2, the copper-clad wall 26 inside theepoxy shell 24 is shown as a circuit board 31 having a copper film 32. APZ element 33 is coupled to the support comprising the epoxy shell 24,circuit board 31 and copper film 32 with a suitable film 34 of cement,such as a cementcommercially available under the trade name Eastman 9l0, orany resin. The cement may be impregnated with conductiveparticles, such as particles of silver, copper or aluminumto provide alow electrical resistance coupling, but that is not necessary since theuse of a resin having a high dielectric constant simply provides ACcoupling of very large capacitance (low impedance).

The PZ element 33 may be made in the form of a disc from a ferroelectricceramic material, such as a suitable barium titanate of lead titanatezirconate, polarized to exhibit a strong piezoelectric effect inresponse to stresses and strains in a direction normal to the wall. Aninertia mass 35 made of a suitable metal (such as copper) ismechanically coupled to the P2 element 33 with a film 36 of cement ofthe same material as the film of cement 34. To complete the assembly ofthe vibration detector 22 in the transducer 24 of FIG. 2, the inertiabody 35 is connected to the inner conductor 28 of the coaxial cable 29as well as the inertia body of the other vibration detectors 21 and 23as shown in F IG. 2.

Once the transducer 20 has been assembled in the manner illustrated anddescribed with reference to FIGS. 2 and 3, it is mechanically coupled tothe resonant cavity of the instrument as shown in FIG. 1 using auniversal wax of a type customarily used for vibration testing, such asa wax commercially available under the trade name Cenco Universal RedWax (Cenco Part No. I 1450) described in the CRC handbook of Chemistryand Physics, 40th Edition, at page 3296. A universal wax is preferredfor mechanically coupling the transducer to the instrument because itcan be readily removed with a suitable solvent, such as xylene, whichviolin makers use to clean instruments.

It has been discovered that, on instruments having resonant cavities,there is at least one vibration point which reflects the same frequencyand tonal qualities of the musical instrument as one hears projecteddirectly to him through the air. The triple-axis transducer 20 iscapable of picking up a maximum of overtones created by the fundamentalnote played on the instrument. These overtones are not governed by thevibration of the string alone, but also by the string interacting withthe resonant cavity functioning as an acoustic chamber. In accordancewith the present invention, the tripleaxis transducer is moved about tofind a point on the musical instrument where the overtones detected bythe transducer 20 are sufficient to provide the same tonal qualitieswhen the electrical signal produced by the transducer 20 is amplified todrive a loudspeaker or other device adapted to convert electric ener gyback into acoustic energy. In other words, by moving the triple-axistransducer 20 about the instrument, one can find a vibration point whichsounds the same through an electronic amplifier as one hears directly(i.e., unamplified). At each point tested, the transducer 20 ismechanically coupled to the instrument with universal wax.

Operation of the transducer 20, when connected to a summing preamplifier40 (FIG. 1) will now be described with reference to the schematicdiagram of FIG. 4 where the internal impedances of the vibrationdetectors 21, 22 and 23 are represented by the respective resistors 41,42 and 43. The internal impedance of a given transducer is very highsince it consists of the resistance of its PZ element, inertial mass anda thin film of coupling cement. However, virtually no current flows intothe input terminal of an operational amplifier 44 connected as anoninverting high input impedance amplifier having differential inputsand with negative feedback through a voltage dividing network comprisingresistors 45 and 46.

Current in the feedback resistors 45 and 46 is the algebraic sum of thecurrents due to input voltages from the vibration detectors 21, 22 and23. Thus, each of the vibration detectors contributes to the totalfeedback current, and therefore the output voltage e Assuming thevoltages produced by the vibration detectors 21, 22 and 23 at a giveninstant of time are e,, e; and e then the output voltage 2 is given bythe following expression:

where R is the resistance of the feedback resistor 45 and R, is theresistance of the bias resistor 46.

It is desirable to employ the operational amplifier 44 as an impedancematching device with an input impedance of over ohms and an outputimpedance in the order of 150 to 200 ohms, to match the high inputimpedance of the transducer with the low input impedance of a poweramplifier 46 (FIG. 1) adapted to drive a loudspeaker 47, or a standardhigh fidelity amplifier, tape recorder, studio microphone or the like.To accomplish that, the noninverting operational amplifier configurationshown is preferred with an overall gain of nominally 10, and a 3-db rolloff at 5 Hz. established by a filter capacitor 48. The frequencyresponse of the entire system is then from 5 Hz. to an upper limit inorder of 100,000 Hz., the upper limit being established by the firstresonant point of the ceramic material used in the transducer. Apotentiometer 49 is provided at the output of the amplifier 44 as avolume control. Thus, the amplifier 40 not only functions as a summingamplifier but also as an impedance matching preamplifier with volumecontrol.

Although a particular embodiment of the invention has been described andillustrated herein, it is recognized that modifications and variationsmay readily occur to those skilled in the art. Consequently, it isintended that the claims be interpreted to cover such modifications andequivalents.

What is claimed is:

l. A method of electronically amplifying sound from a vibration point ina musical instrument selected from various points that are subject tovibrations in three orthogonal axes to produce for a given note afundamental note played and overtones that provide the tonal qualitiesof said instrument, comprising the steps of:

attaching a transducer to said instrument at one of said various points,said transducer having three vibration detectors, a separate detectorfor vibrations along each of three orthogonal axes;

adding signals proportional to vibrations in said three axes produced bysaid three detectors of said transducer in response to vibrations ofsaid instrument at said one vibration point to produce a sum signal;

amplifying said sum signal to produce an amplified signal;

applying said amplified signal to a device for converting said sumsignal into audible sound;

moving said transducer about said instrument to said various points andattaching said transducer to said instrument at each point until anoptimum vibration point is found where sound produced by said devicefrom said amplified signal sounds the same as unamplified sounddirectly; and

leaving said transducer at said optimum point for amplification of soundthereafter.

2. A method as defined in claim 1 wherein said instrument has a resonantcavity and said optimum vibration point is on a wall forming saidcavity.

3. On a musical instrument, a transducer that responds to vibrations ofsaid instrument in three orthogonal axes for producing across a pair ofoutput terminals of said transducer an electrical signal that is the sumof separate electrical signals, each proportional to vibrations in adifferent one of said three axes, comprising:

three separate means for detecting vibrations, each adapted to detectvibrations along a different one of said three axis and produce avoltage signal proportional thereto;

a common support means for said detecting means, said support meansbeing adapted to secure each detecting means for detection of vibrationsin a different one of said three axes;

means having a pair of input terminals for summing signals applied inparallel across said input terminals to produce a sum signal;

means for connecting said three detecting means in parallel between saidinput terminals of said summing means; and

means for amplifying said sum signal produced by said summing means. 4.A combination as defined in claim 3 wherein said support means com gisesa sealed enclosure filled with siliconrubber.

5. A com matron as defined in claim 4 wherein said enclosure is linedwith a film of conductive material to form an electrostatic shield, andincluding a coaxial cable having an outer conductor connected tosaidfilm and to one terminal of each of said vibration detecting means toform one output terminal of said pair, and an inner conductor connectedto another terminal of each of said vibration detecting means to form asecond output terminal of said pair.

6. A combination as defined in claim 5 wherein each vibration detectingmeans comprises a piezoelectric element having a pair of parallel faces,a first one cemented to said film, and an inertia mass of conductivematerial cemented to a second one of said pair of parallel faces,whereby an area of said film to which each element is cemented providesone terminal of each of said vibration detecting meansand said inertiamass of each of said vibration detecting means provides the otherterminal thereof.

7. A combination as defined in claim 6 wherein said summing meanscomprises a noninverting operational amplifier connected to saiddetecting means in parallel by said coaxial cable, said noninvertingoperational amplifier having differential input terminals and an outputterminal, a bias resistor connected between one of said differentialinput terminals and said outer conductor of said coaxial cable, afeedback resistor connected between said amplifier output terminal andsaid one of said differential input terminals and another of saiddifferential input tenninals connected to said inner conductor of saidcoaxial cable.

8. On a musical instrument, a triple-axis transducer that responds tovibrations of said instrument for producing across a pair of outputterminals an output signal that is the sum of three signals, each ofsaid three signals being proportional to vibrations in a different oneof three orthogonal axes, comprismg:

a common support having three orthogonal conductive walls;

three vibration detectors, each mounted on a unique one of said threewalls, and each comprising a piezoelectric element having a pair ofparallel faces, a first one of said faces being cemented to a conductivewall, and an inertia mass cemented to a second one of said faces; and

a flexible conductor connecting a first one of said output terminals tosaid inertia mass of each one of said vibration detectors, and meansconnecting said conductive walls to a second one of said outputterminals.

9. A combination as defined in claim 8, wherein port is a sealedenclosure filled with silicon rubber.

10. A combination as defined in claim 9, wherein all walls of saidsealed enclosure support are conductive to form an electrostatic shieldaround said detectors.

11. A combination as defined in claim 10, wherein all conductive wallsare comprised of a film of conductive material on a board of dielectricmaterial, and all films of all walls are connected togetherelectrically.

12. A combination as defined in claim 11, wherein said first one of saidoutput terminals is connected to an inner conductor of a coaxial cable,and an outer conductor of said coaxial cable is connected to saidconductive walls to form said second output terminal.

13. A combination as defined in claim 12, wherein said sealed enclosuresupport is completely coated on the outside with dielectric andwaterproof material to provide an electrical and moisture seal aroundsaid outer conductor of said coaxial cable.

said sup-

1. A method of electronically amplifying sound from a vibration point ina musical instrument selected from various points that are subject tovibrations in three orthogonal axes to produce for a given note afundamental note played and overtones thaT provide the tonal qualitiesof said instrument, comprising the steps of: attaching a transducer tosaid instrument at one of said various points, said transducer havingthree vibration detectors, a separate detector for vibrations along eachof three orthogonal axes; adding signals proportional to vibrations insaid three axes produced by said three detectors of said transducer inresponse to vibrations of said instrument at said one vibration point toproduce a sum signal; amplifying said sum signal to produce an amplifiedsignal; applying said amplified signal to a device for converting saidsum signal into audible sound; moving said transducer about saidinstrument to said various points and attaching said transducer to saidinstrument at each point until an optimum vibration point is found wheresound produced by said device from said amplified signal sounds the sameas unamplified sound directly; and leaving said transducer at saidoptimum point for amplification of sound thereafter.
 2. A method asdefined in claim 1 wherein said instrument has a resonant cavity andsaid optimum vibration point is on a wall forming said cavity.
 3. On amusical instrument, a transducer that responds to vibrations of saidinstrument in three orthogonal axes for producing across a pair ofoutput terminals of said transducer an electrical signal that is the sumof separate electrical signals, each proportional to vibrations in adifferent one of said three axes, comprising: three separate means fordetecting vibrations, each adapted to detect vibrations along adifferent one of said three axis and produce a voltage signalproportional thereto; a common support means for said detecting means,said support means being adapted to secure each detecting means fordetection of vibrations in a different one of said three axes; meanshaving a pair of input terminals for summing signals applied in parallelacross said input terminals to produce a sum signal; means forconnecting said three detecting means in parallel between said inputterminals of said summing means; and means for amplifying said sumsignal produced by said summing means.
 4. A combination as defined inclaim 3 wherein said support means comprises a sealed enclosure filledwith silicon rubber.
 5. A combination as defined in claim 4 wherein saidenclosure is lined with a film of conductive material to form anelectrostatic shield, and including a coaxial cable having an outerconductor connected to said film and to one terminal of each of saidvibration detecting means to form one output terminal of said pair, andan inner conductor connected to another terminal of each of saidvibration detecting means to form a second output terminal of said pair.6. A combination as defined in claim 5 wherein each vibration detectingmeans comprises a piezoelectric element having a pair of parallel faces,a first one cemented to said film, and an inertia mass of conductivematerial cemented to a second one of said pair of parallel faces,whereby an area of said film to which each element is cemented providesone terminal of each of said vibration detecting means and said inertiamass of each of said vibration detecting means provides the otherterminal thereof.
 7. A combination as defined in claim 6 wherein saidsumming means comprises a noninverting operational amplifier connectedto said detecting means in parallel by said coaxial cable, saidnoninverting operational amplifier having differential input terminalsand an output terminal, a bias resistor connected between one of saiddifferential input terminals and said outer conductor of said coaxialcable, a feedback resistor connected between said amplifier outputterminal and said one of said differential input terminals and anotherof said differential input terminals connected to said inner conductorof said coaxial cable.
 8. On a musical instrument, a triple-axistransducer that responds to vibrations of said instRument for producingacross a pair of output terminals an output signal that is the sum ofthree signals, each of said three signals being proportional tovibrations in a different one of three orthogonal axes, comprising: acommon support having three orthogonal conductive walls; three vibrationdetectors, each mounted on a unique one of said three walls, and eachcomprising a piezoelectric element having a pair of parallel faces, afirst one of said faces being cemented to a conductive wall, and aninertia mass cemented to a second one of said faces; and a flexibleconductor connecting a first one of said output terminals to saidinertia mass of each one of said vibration detectors, and meansconnecting said conductive walls to a second one of said outputterminals.
 9. A combination as defined in claim 8, wherein said supportis a sealed enclosure filled with silicon rubber.
 10. A combination asdefined in claim 9, wherein all walls of said sealed enclosure supportare conductive to form an electrostatic shield around said detectors.11. A combination as defined in claim 10, wherein all conductive wallsare comprised of a film of conductive material on a board of dielectricmaterial, and all films of all walls are connected togetherelectrically.
 12. A combination as defined in claim 11, wherein saidfirst one of said output terminals is connected to an inner conductor ofa coaxial cable, and an outer conductor of said coaxial cable isconnected to said conductive walls to form said second output terminal.13. A combination as defined in claim 12, wherein said sealed enclosuresupport is completely coated on the outside with dielectric andwaterproof material to provide an electrical and moisture seal aroundsaid outer conductor of said coaxial cable.